We describe a lab-scale closed-circulating test system for photocatalytic wastewater treatment. The system comprises a UV-LED photoreactor, a microcirculating fluid pump, and an in-stream sensor unit. The reactor can hold volumes up to 250 mL and is optimized to study the degradation of pollutant concentrations in the microgram to milligram per liter range using photocatalysts fixed to a planar surface within the reactor vessel. The test pollutant used was methyl orange. The in-stream sensor unit consists of a liquid flow cell with transparent windows, allowing the transmission of light from an LED to be monitored by a photodiode. The concentration of the pollutant is evaluated in real-time. The system is lightweight, cheap, portable, and flexible, ideal for laboratory or fieldwork use, and could be easily up-scaled and used for in-line quality control monitoring in a wastewater treatment plant.